1. Field of the Invention
The present invention relates to a control apparatus having a non-volatile random access memory (RAM), central processing unit (CPU) and protecting portion intervening between RAM and CPU.
2. Description of the Related Art
Recently, in a system having non-volatile RAM (Random Access Memory), the non-volatile RAM is sometimes required to be divided into ROM (Read Only Memory) regions and RAM regions. For example, Japanese Patent Laid-Open Publication No, 1995-114497 discloses a system having the non-volatile RAM, CPU and a protecting portion. The protecting portion (i.e., bus control circuit) intervenes between the RAM and CPU so as to prohibit writing to a region defined as the ROM.
In this configuration, a register arranged in the protecting portion includes address information which specifies the ROM region and an access mode, i.e., writing data to the ROM region is detected based on the address information to prohibit write access to the ROM region.
In the above-described configuration, data such as program data to be used in the CPU are stored in the ROM region. This type of data is preferably renewed when it is necessary. To renew the data, for instance, it is considered that writing data to the ROM region is allowed only when an external signal is inputted.
However, when such an external signal is being allowed to enter the system, unintended signals such as noise may enters as well. In this regard, depending on the signal, the address information could possibly be changed. If the address information is changed, e.g. a part of the ROM region is recognized erroneously as a RAM region, unintended access, i.e., writing data to the ROM region may occur.
In the above-described configuration, it is convenient that the data stored in the ROM region is renewed. However, the above-described problem is likely to happen.